The present application pertains to the field of RFID systems operating at 13.56 MHz, which are usually referred to as high-frequency (HF) RFID. More specifically the generation of a response by an RFID transponder using active load modulation, ALM, is addressed. When using ALM a transponder actively generates a load modulated signal, thereby emulating the standard passive load modulation described, e.g., in ISO/IEC 14443 standard. ALM is employed instead of passive load modulation whenever the signal generated by passive load modulation is not strong enough to be detected by an RFID interrogator or reader device. This may be the case when transponder antennas are small or are located in a challenging environment, for example.
Transponders featuring ALM are required to actively generate and transmit a burst at a carrier frequency identical to the carrier frequency of an interrogating device. This means that each burst of the generated ALM carrier signal starts with the same phase difference to the carrier signal emitted by the interrogating device. Unchanged phase also implies that the frequencies of the ALM carrier signal and the carrier signal of the interrogating device are identical.
In state of the art implementations constant phase difference between the ALM carrier signal and interrogator carrier signal is achieved in two ways.
One possibility is to observe the interrogator carrier frequency before the transmission of the ALM carrier signal starts and to adjust a frequency source in the ALM transponder to substantially the same frequency. A complete transmission sequence is called a frame and is composed of series of ALM carrier bursts separated by gaps without ALM carrier emission. Once the transmission of a frame starts, the transponder frequency source is used to generate the ALM carrier frequency. This frequency source is occasionally re-adjusted during the gaps in the frame. During these gaps only the interrogator carrier signal is present on a transponder antenna, which is used as a reference frequency to re-adjust the ALM carrier frequency source. This possibility is called intra-frame synchronization.
According to a second possibility, the ALM carrier frequency source is adjusted once during the time before the ALM transmission of a frame starts and the frequency source is not re-adjusted during the frame transmission. This means the initial adjustment has to be extremely precise and stable so that the phase change during the longest frame transmission is below a limit defined in the standard, e.g., ISO 14443. The stability of the ALM carrier frequency source can thus be maintained only in case it is controlled or kept stable by an external high quality frequency reference. For this possibility the initial frequency measurement is effected, for instance, by using two phase-locked loops, PLLs, where one PLL is imbedded in another PLL. The second possibility is called inter-frame synchronization.